LANHAM, Md. (CBSDC/AP) — Nearly 40 years after two girls disappeared walking home in Montgomery County, police say they have a potential break in the case.

The Lyon sisters — Sheila, 12, and Katherine, 10 — vanished on March 25, 1975 while walking home from Wheaton Plaza in Kensington.

Photo credit: Montgomery County Police Department

Several witnesses, including the girls’ older brother, saw them inside the shopping mall before their disappearance. An extensive search and investigation resulted, but no one has ever been charged in the case.

Police say detectives have now identified a person of interest, 57-year-old Lloyd Lee Welch, who was at Wheaton Plaza on the day the girls disappeared. Welch is a convicted child sex offender who has been in a Delaware prison since 1997.

But the case is based so far on circumstantial rather than physical evidence, which is why police say they’re seeking the public’s help to learn more about the man and his activities as they try to assemble enough information for a potential criminal prosecution.

“If we were able to charge somebody, we’d have done it,” county Police Chief J. Thomas Manger said at a news conference.

Photo credit: Montgomery County Police

Authorities say Welch traveled extensively throughout the U.S. from the 70s through the mid-90s. He worked as a ride operator for a carnival company, often set up at malls. He was arrested in several states, charged with sexual offenses against young girls. Police believe there may be victims who have not come forward.

An attorney for Welch could not immediately be found. A lawyer who previously represented him in a Delaware child sex assault case did not immediately return a call seeking comment.

In a statement distributed by police Tuesday, the Lyon family thanked the public for its continued interest but requested privacy.

“The fact that so many people still care about this case means a great deal to us,” the family said.

“We have all been haunted by the disappearance of the Lyon sisters. Even though so much time has passed, we have not forgotten that those young girls deserve justice, and their family deserves closure. We’re hoping one of you has information that will help us provide both,” Manger said Tuesday.

Police on Tuesday released a timeline including about two dozen U.S. locations, from California to Iowa to South Carolina, where Welch is believed to have visited between 1974 and 1997. Authorities are investigating whether Welch, who is originally from Washington, D.C., and was arrested for burglary near the mall in the mid-1970s, committed other crimes in those places.

He was known to hitchhike in and around the region, spent time in hotels and homeless shelters and worked in different locations as a landscaper and for a carnival company that often set up at malls, police said.

“We understand we’re asking people to think back several decades to remember details they may not think matter. However, the information you provide could be vital in our search for evidence, and put to rest difficult questions for the Lyon family and for any other victims who may exist,” said Steve Vogt, Special Agent in Charge of the FBI in Maryland.

This isn’t the first time that police have singled out a person of interest.

In 1987, for instance, detectives pursued a possible link involving a convicted child molester imprisoned in North Carolina but did not bring any charges. That same year, the police said the girls had been seen at the mall with a man who had a briefcase containing a tape recorder, according to news accounts. Manger said Welch is not believed to be the same person.

He said the department was committed to finding justice for the family.

“These girls deserve a proper burial, frankly,” Manger said.

Anyone who may have encountered Welch is asked to call investigators at 1-800-CALL-FBI, or they can submit a tip online at http://tips.fbi.gov.

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